People dressed as zombies take part in a demonstration against drugs in the streets of downtown Guatemala City. Reuters

Synthetic cannabis – also referred to as "fake pot" – is behind a "zombie outbreak" in the US, a medical journal has revealed.

The legal high sold in shiny gold packages with the word "spice" emblazoned across it was leaving its users glassy-eyed, aimless and sprawled on the streets.

After more than 30 people were believed to have overdosed on the substance in New York City in the space of 11 hours this July, researchers confirmed synthetic cannabinoids were responsible.

Researchers at The New England Journal of Medicine have now confirmed the substance was 85 times more potent than regular marijuana.

Spice was also having a profound effect on homeless populations.

Los Angeles, California; Austin, Texas; and St Louis, Missouri were among the worst-affected cities, reported ABC News.

The Reverend Larry Rice, who runs the New Life Evangelistic Center in downtown St Louis, told the news outlet that the drug was used by those who wanted a cheap escape from the harsh reality of life on the streets.

"They told me, 'You get so low, you get such a sense of hopelessness. Somebody wants to sell this for a dollar and you take it,'" Rice said. "People are desperate out there."

It was also more difficult to detect in drug tests and was odourless, making it less likely to be caught by enforcement officials.

The Drug Enforcement Agency and several state legislatures have outlawed variations of the "fake pot" however suppliers continually tweaked the formula to maintain its legality.

However, these changes are believed to be causing the adverse reactions, which bystanders have often described as "zombie-like".

Lieutenant Kurt Thomas, of Austin Police Department, said: "It was common for us to see reactions where they were violent, incoherent, [and] sometimes catatonic on the ground."

The drug has caused similar problems in the UK, particularly among prison populations, but the government placed a blanket ban on Spice, and similar new psychoactive substances, in May this year.